The present invention relates to a screw drum type filter for filtering sludge containing suspended particles to separate it into filtrate and dehydrated cake.
As for conventional screw drum type filters, there is, for example, a through-screw type filter cylinder installation system, as disclosed in Japanese Patent No. 1520106. A filter of this system comprises a filter cylinder installed in a substantial range in a drum excluding the opposite ends of the drum for filtering sludge radially from inside to outside of the cylinder, a number of through-holes formed in the peripheral surface of the drum in said substantial range, a screw installed throughout the length of the drum to extend through said filter cylinder, the spaces in said drum at opposite ends thereof being used as an inlet chamber for sludge and an outlet chamber for dehydrated cake. The filtration passage in the filter cylinder is defined by small clearances between alternating annular fixed and movable plates that constitute said cylinder.
In the above arrangement, in order to swing the movable plates, a shaft equipped with a cam key is fitted in circular holes formed in the lower ends of the movable plates and is eccentrically driven while supporting the weight of the plates while a fulcrum bar is loosely fitted in separate circular holes formed in the upper ends of the movable plates to effect vertical and transverse positional control. However, in such arrangement with the cam-key-equipped shaft disposed at a position corresponding to the lower ends of the movable plates, the water forced out from between the plates of the filter cylinder (which water still contains some suspended particles) flows down onto the shaft and then into the bearings at the opposite ends to wear them.
The opening edge shapes of the fixed and movable plates are generally triangular-wavelike serrations, rather than typical round shapes, formed along the circular base lines so as to increase the area of contact with sludge; with such arrangement, however, the suspended particles in the sludge to be processed accumulate in the triangular wavelike troughs of the opening edges, weakening the filtration effect between plates and, in some cases, damaging the vane edges of the screw.
A first object of the present invention is to provide a screw drum type filter wherein a cam or shaft means for direct contact drive of the movable plates of a filter cylinder so as to swing the movable plates is placed at a position sufficiently remote from the middle lower portion of the filter cylinder to prevent the filtrate water forced out from the clearances between the plates from falling down onto said means in large amounts.
A second object of the invention is to provide a screw drum type filter wherein the opening edges of the fixed and movable plates are shaped such that suspended particles hardly accumulate while the area of contact with sludge to be processed is maintained increased to some degree.
To achieve the first object, a screw drum filter according to the invention comprises
a) a filter cylinder composed of an approximately horizontally extending alternately superposed array of a number of annular fixed plates having circular or approximately circular openings, and a number of annular floating plates having geometrically similar openings slightly larger in diameter than the openings in said fixed plates and also having circular outer peripheries, with small filtration clearances defined, the row of openings in said fixed plates defining a cylindrical space having a fixed inner peripheral contour, it being arranged that when the row of openings in said floating plates is positioned outside said cylindrical space, at least two equi-level regions in the lower portion of the outer periphery of the floating plate are exposed beyond the outer edge of the fixed plate,
b) a screw (2) extending through said cylindrical space for compression-feeding sludge to be processed in a direction with one end of the cylindrical space in said filtration cylinder as the inlet end and the other end as the outlet end,
c) at least two eccentric shafts contact-wise supporting said exposed outer peripheral portions of said floating plates and eccentrically rotated, so as to swing the floating plates, and
d) a driving mechanism for operatively connecting said screw pitch and said eccentric shafts or for individually driving them.
According to the above arrangement, each floating plate is contacted and supported by the eccentric shafts at the equi-level positions in its peripheral edge sufficiently separated from the middle lower portion thereof; therefore, the water forced out through the filtration clearances between the plates mostly moves along the outer peripheral edges of the movable and fixed plates to reach the lower portions of the plates below the cylindrical shaft, whereupon it flows down, with the result that it hardly collects on the eccentric shafts disposed right and left and the bearings are sufficiently protected against entry of the filtrate water. In addition, the term floating plate is used in almost the same meaning as that of the term movable plate in the conventional filter, and the floating plate is named in consideration of the function of producing a rotation whose speed transmission ratio varies.
Further, to achieve the second object, the screw drum filter according to the invention is such that the opening contours of the fixed and floating plates are defined by rounded wavelike closed curves along the circular base lines.